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  #11  
Old 09-14-2012
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Re: Propeller Drag - Challenging my intuition

I'd be interested in an answer as well - I'm less concerned with drag as I am wear on the engine / transmission / stuffing box.

I usually sail in neutral.
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Old 09-14-2012
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Re: Propeller Drag - Challenging my intuition

Quote:
Originally Posted by johnnyquest37 View Post
Drag is not the only issue. Preventing damage to the transmission left spinning is the primary reason for preventing the prop from spinning when the engine is off.
That is a valid point. Yanmar seems to think that locking the prop causes more damage. Yet, I figure, rotating a bunch of metal at uncontrolled RPM for long periods of time is likely to damage more than locking it. Again, my gut-feeling could be wrong there (and Yanmar says it is.)
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Old 09-14-2012
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Re: Propeller Drag - Challenging my intuition

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Originally Posted by peoples1234 View Post
Leaving it in neutral and letting the prop spin was much less resistance than not allowing the prop to spin freely. A three blade prop that was not allowed to spin caused as much drag as dragging a 5 gal bucket through the water.
Fair enough. I'm willing to be convinced by science and experimentation.

What about my case of 2-blade prop that can align with my full keel?

I think the folding-prop case is a no-brainer though. It has gotta be locked or the prop won't fold.

As for wear and tear ... the spinning prop and shaft don't cause wear & tear?? I'd think that'd cause lots.
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Old 09-14-2012
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Re: Propeller Drag - Challenging my intuition

Yeah - looks like I cited the same website as the OP. Regardless, a free-spinning prop clearly presents significantly less resistence that one locked in place. Some transmissions or coupling systems are better off not free-wheeling, and some prefer not to be subjected to the forces created from the locked down propeller drag. One has to read the manufacturers' suggestions.
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Old 09-14-2012
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Re: Propeller Drag - Challenging my intuition

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Originally Posted by JordanH View Post
I'm not convinced the wheel analogy is apropos since the 'drag' caused by a locked wheel is high because of friction.

Another point I read in a link was that if you have a folding prop (I don't) then free-wheeling the prop means the blades will/may stay open and cause drag instead of folding and getting out of the way. Clearly there are other variables and considerations to take into consideration.
Another way to look at it is that you're saying that putting the prop in neutral allows it to spin and spinning may cause drag. So basically that there is a prop speed I'll call X which increases drag.

But think about turning on the motor while sailing. At some point while increasing the throttle you'll hit prop speed X, so do you still think this would slow the boat down more than not running the engine at all? I don't think this is consistent with experience.

I agree that folding props add a level of complication (and there are always extra complications with fluids). However I don't see why the blades would necessarily fold out of left in neutral. The drag from the water would still be trying to push the blades into a folded position. Perhaps the prop would spin a bit while still remaining folded. I think this would be a good thing. Of course if the blades did fold out I agree that would probably be bad.
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Re: Propeller Drag - Challenging my intuition

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Originally Posted by JordanH View Post
That is a valid point. Yanmar seems to think that locking the prop causes more damage. Yet, I figure, rotating a bunch of metal at uncontrolled RPM for long periods of time is likely to damage more than locking it. Again, my gut-feeling could be wrong there (and Yanmar says it is.)
I agree with your gut on this one. I'm a bit surprised that it's better not to lock it. If it really does lock it, and I assume it does then there should be no movement. And for mechanical things that's nearly always good. Perhaps the force of water is significant and flexes and stresses the transmission as the boat hits waves etc. Although I fail to see how this is much different when the engine is actually driving the boat.
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Re: Propeller Drag - Challenging my intuition

An MIT study referenced by the OP's citation estimated that prop speed of a free-wheeling propeller was 95% of the prop speed required to move a boat at the same speed under power. So, whether sailing or motoring, the free-wheeling propeller is spinning at essentially the same rate. If this is the case, the folding prop might not stay folded. Besides, the whole point in getting a folding prop is to reduce drag when under sail and not have the shaft spinning. If you are going to let your prop spin, I don't see the reason for having a folding prop.
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Re: Propeller Drag - Challenging my intuition

After reading a number of aeroplane related threads, it looks like there is a dependency on the pitch of the prop as well as the drag of the shaft.

For example, with a hypothetical pitch of a propeller facing diretly forwards (face of prop perpendicular to the water), it would slice through the water effectively and have little drag, but as the pitch changes and you present more and more flat surface to the water, you increase the drag. Additionally, you have the drag of rotating the shaft since a propeller isn't truly freewheeling in a frictionless world.

Fair enough, looks like I have to change my thinking on this.

I still think it might be more effective for me if I could find a method of ensuring my prop is locked vertically instead of just free-wheeling. Any ideas?
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Old 09-14-2012
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Re: Propeller Drag - Challenging my intuition

Yanmar did post an official notice not too long ago about this subject and recommended that it should be left in neutral.

The old recommendation was to leave the engine in reverse to lock the prop in place. This is a quote from a another thread I read on the subject explaining Yanmar's recommendation change - "Apparently the cones when locked can vibrate or chatter against each other, causing wear to the cone surfaces, which then leads to slipping, which then leads to a re-built gear box (I am paraphrasing here). For a while they said it was okay but as the numbers of issues started growing they adamantly changed their requirement."
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Old 09-14-2012
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Re: Propeller Drag - Challenging my intuition

Quote:
Originally Posted by JordanH View Post
If you've ever fished with a spinning lure on the end of your line you'll notice the significant force that it takes to drag that lure through the water. You can feel the reduction in drag immediately should the spinner get hung-up and stop rotating. Your line immediately goes light and you can reel in quite quickly.

My gut feeling is from observation of dragging many a Mepps through the water.
Perhaps this will help:




My challenge is still open to any "non-believers" pay for my time and we can test props all day long. When we are done you need to show your mug on You Tube eating non-believing crow..... So far not a single taker....
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Last edited by Maine Sail; 09-14-2012 at 04:58 PM.
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